Centrifugal rotating machine

ABSTRACT

A centrifugal rotating machine includes a rotating shaft, an impeller, the impeller including a disk fixed to the rotating shaft and a cover covering a blade provided on the disk, and a casing which accommodates the impeller. An impeller flow path through which the fluid is pumped is formed by an upstream surface of the disk in the axial direction and an inner peripheral surface of the cover. In addition, an outer flow path is formed by an outer peripheral surface of the cover and an inner peripheral surface of the casing facing the outer peripheral surface of the cover. The outer flow path is connected to the impeller flow path at an outlet of the impeller flow path, and a protrusion portion protruding from the inner peripheral surface of the casing is provided in the outer flow path.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a centrifugal rotating machine.

Priority is claimed on Japanese Patent Application No. 2018-241697, filed on Dec. 25, 2018, the content of which is incorporated herein by reference.

Description of Related Art

In general, a centrifugal compressor has a rotating shaft which extends along an axis, an impeller which is provided on the rotating shaft, and a casing which covers the impeller from an outside. Among these, in the impeller, there is an impeller referred to as a closed impeller. The closed impeller has a disk-shaped disk centered on an axis, a plurality of blades which are provided on one surface of the disk, and a conical cover which covers the plurality of blades from one side. A clearance (outer flow path) is provided between an outer peripheral surface of the cover and an inner peripheral surface of the casing.

If the centrifugal compressor is operated, a fluid flows through a flow path defined by the blades. The fluid is compressed and brought into a high pressure state while flowing through the flow path from an inlet side toward an outlet side of the flow path. Here, the fluid having a pressure higher than that of the inlet side flows through the outlet side of the flow path, and thus, the fluid flows into the above-described outer flow path. Accordingly, in a case where lot of fluid flows into the outer flow path, compression efficiency of the centrifugal compressor decreases. Therefore, a technology is known in which a seal portion for preventing the flow of the fluid is provided on the inner peripheral surface of the casing. For example, PCT International Publication No. WO 2016/043090 discloses a configuration in which a seal fin is provided on the inlet side of the impeller in the inner peripheral surface of the casing as a specific example of the seal portion. By providing the seal fin, the fluid which flows into the outer flow path is reduced.

In the centrifugal compressor having the above configuration, if a rotor including the impeller is displaced in a radial direction in a state where the fluid leaks between the seal fin and the outer peripheral surface of the cover, a circumferential pressure distribution is generated on a rotor surface. Here, a swirl component (swirling flow component) according to the rotation of the impeller is added to the fluid flowing through the outer flow path. Due to influences of this swirl component, an excitation force (seal excitation force) in a direction orthogonal to the displacement direction acts on the impeller. This seal excitation force is continuously applied, and thus, a swing-rotation vibration is generated in the rotor. That is, the centrifugal compressor described in PCT International Publication No. WO 2016/043090 still has a room for improvement.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-described problems, and an object thereof is to provide a centrifugal rotating machine in which a vibration is further reduced.

According to an aspect of the present invention, there is provided a centrifugal rotating machine including: a rotating shaft which extends along an axis; an impeller which is configured to pump a fluid flowing in from one side in an axial direction to an outside in a radial direction, the impeller including a disk fixed to the rotating shaft and a cover covering a blade provided on the disk; and a casing which accommodates the impeller, in which an impeller flow path through which the fluid is pumped is formed by an upstream surface of the disk in the axial direction and an inner peripheral surface of the cover, an outer flow path is formed by an outer peripheral surface of the cover and an inner peripheral surface of the casing facing the outer peripheral surface of the cover, the outer flow path is connected to the impeller flow path at an outlet of the impeller flow path, and a protrusion portion protruding from the inner peripheral surface of the casing is provided in the outer flow path.

According to the configuration, the fluid which has flowed into the outer flow path is guided by the protrusion portion provided on the inner peripheral surface of the casing. Therefore, even in a case where the fluid includes a swirl component (swirling flow component), the fluid is guided by the protrusion portion, and thus, the swirl component can be reduced. Here, in a case where the fluid including the swirl component flows into the outer flow path, an excitation force (seal excitation force) in a direction orthogonal to a displacement direction acts on the impeller. This seal excitation force is continuously added, and thus, a swing-rotation vibration is generated in the rotating shaft and the impeller. However, according to the configuration, the possibility of the above-described problems can be reduced.

In the centrifugal rotating machine, a plurality of the protrusion portions may be formed at intervals in a circumferential direction.

According to the above configuration, the plurality of protrusion portions are formed at intervals in the circumferential direction. Therefore, the swirl component can be uniformly reduced over the entire area of the outer flow path in the circumferential direction. Thereby, the pressure distribution of the fluid in the outer flow path becomes uniform, and thus, vibrations generated in the impeller can be further effectively suppressed.

In the centrifugal rotating machine, the protrusion portion may be provided at a predetermined region on the inner peripheral surface of the casing.

In the centrifugal rotating machine, the protrusion portion may be provided at a position overlapping the outlet of the impeller flow path in a radial direction with respect to the axis.

According to the configuration, the protrusion portion is provided at the position overlapping the outlet of the impeller flow path in the radial direction. Therefore, the swirl component included in the fluid which has flowed into the outer flow path can be reduced immediately after the inflow. Here, the present inventors performed a CFD analysis on the excitation force by the swirl component. As a result, it was found that the excitation force generated in the impeller cover was large. The excitation force generated in this cover is caused by the fluid flowing into the outer flow path. Accordingly, as described in the configuration, the protrusion portion is provided at the position overlapping the inlet of the outer flow path, that is, the outlet of the impeller flow path. Therefore, the swirl component is reduced, and the excitation force generated in the cover can be more positively reduced. In addition, the swirl component increases as it gets closer to the outlet of the impeller flow path. Therefore, the swirl component can be more effectively reduced. As a result, it is possible to further reduce a possibility of the impeller being displaced or excited due the influence of the swirl component. Further, as compared to a configuration in which the protrusion portion extends over the entire area of the outer flow path, only a necessary and sufficient amount of the swirl component is removed. Accordingly, it is also possible to avoid an increase in a frictional resistance between the cover outer peripheral surface of the impeller and the fluid due to an excessive reduction of the swirl component.

The centrifugal rotating machine may further include a step which is provided on the outer peripheral surface of the cover and is an annual step centered on the axis in the outer flow path, in which the step may be provided radially outside the protrusion portion.

According to the above configuration, the step is provided on the outer peripheral surface of the cover. Therefore, the fluid flowing out from the outlet of the impeller passes through a radially outer portion of the outer flow path. That is, more fluid flows along the inner peripheral surface of the casing where the protrusion portion is provided. As a result, more fluid is guided toward the protrusion portion provided on the inner peripheral surface of the casing. Thereby, the swirl component of the fluid which flows into the outer flow path can be more positively reduced.

In the centrifugal rotating machine, the protrusion portion may be provided over the entire area of the casing inner peripheral surface in the outer flow path.

According to the configuration, since the protrusion portion is provided over the entire area of the outer flow path, the swirl component of the fluid which has flowed into the outer flow path can be further reduced.

In the centrifugal rotating machine, the protrusion portion is curved from one side toward the other side in a circumferential direction with respect to the axis as the protrusion portion goes outward in the radial direction.

Here, the swirl component which turns from the other side toward the one side in the circumferential direction (that is, toward front side in rotation direction of the impeller) is added to the fluid flowing into the outer flow path. According to the above configuration, the protrusion portion is curved from the one side toward the other side in the circumferential direction as the protrusion goes radially outward from the inlet side. That is, the protrusion portion is curved in a direction opposite to a turning direction of the swirl component. Therefore, the swirl component can be rectified in the opposite direction by the protrusion portion. As a result, it is possible to further reduce the possibility of the rotating shaft and the impeller being excited due to the influence of the swirl component.

In the centrifugal rotating machine, the protrusion portion may be twisted from one side toward the other side in a circumferential direction with respect to the axis with reference to the inner peripheral surface of the casing as the protrusion portion goes from the outlet side toward the inlet side.

According to the above configuration, since the twist of the protrusion portion is small on the outlet side, the protrusion portion has a large angle with respect to the inner peripheral surface of the casing. Therefore, the flow of the fluid including the swirl component which has flowed into the outer flow path from the outlet side (that is, upstream side of outer flow path of the impeller can be captured more efficiently. Thereby, the swirl component can be further reduced.

The centrifugal rotating machine may further include a seal portion which is provided on a radially inner end portion of the outer flow path and seals leakage of the fluid between the inner peripheral surface of the casing and the outer peripheral surface of the cover.

According to the above configuration, since the twist of the protrusion portion is large on the inlet side (that is, downstream side of outer flow path) of the impeller, the protrusion portion has a small angle with respect to the inner peripheral surface of the casing. Therefore, the fluid guided by the protrusion portion flows around the inner peripheral surface of the casing. As a result, for example, in a case where the seal portion is provided on the upstream side of the casing inner peripheral surface, the fluid can flow more positively toward the seal portion itself rather than toward the clearance between the seal portion and the cover outer peripheral surface of the impeller. That is, the flow into the seal portion on the inlet side of the impeller becomes a radially inward flow state (down flow) from the casing inner peripheral surface to the cover outer peripheral surface, and thus, a contraction effect in the seal portion increases. Thereby, a leak flow which passes through the seal portion can be further reduced.

In the centrifugal rotating machine, the protrusion portion may be twisted from a front side toward a rear side in a rotation direction of the impeller with reference to the inner peripheral surface of the casing as the protrusion portion goes inward in the radial direction.

According to the above configuration, the protrusion portion is twisted toward a rear side (that is, side opposite to turning direction of swirl component included in flow in outer flow path) in a rotation direction of the impeller. Accordingly, it is possible to more effectively capture and reduce the swirl component.

In the centrifugal rotating machine, a circumferential dimension of the protrusion portion with respect to the axis gradually decreases as the protrusion portion is separated from the inner peripheral surface of the casing.

According to the above configuration, the protrusion portion has a tapered shape such that a dimension thereof in a circumferential direction gradually decreases. Accordingly, for example, even in a case where the outer peripheral surface of the impeller comes into contact with the protrusion portion, a contact area between the protrusion portion and the outer peripheral surface can be suppressed to be small. As a result, it is possible to prevent the impeller from being damaged or from being vibrated.

According to another aspect of the present invention, there is provided a centrifugal rotating machine including: a rotating shaft which extends along an axis; an impeller which is configured to pump a fluid flowing in from one side in an axial direction to an outside in a radial direction, the impeller including a disk fixed to the rotating shaft and a cover covering a blade provided on the disk; and a casing which accommodates the impeller, in which an impeller flow path through which the fluid is pumped is formed by an upstream surface of the disk in the axial direction and an inner peripheral surface of the cover, an outer flow path is formed by an outer peripheral surface of the cover and an inner peripheral surface of the casing facing the outer peripheral surface of the cover, the outer flow path is connected to the impeller flow path at an outlet of the impeller flow path, and a step which is an annual step centered on the axis is provided on the outer peripheral surface of the cover in the outer flow path.

According to the above configuration, since the step is provided on the outer peripheral surface of the cover, the gap between the outer peripheral surface of the cover and the inner peripheral surface of the casing can be reduced. That is, the amount of fluid flowing into the outer flow path can be limited. As a result, it is possible to reduce the excitation force to the impeller generated when a large amount of fluid flows into the outer flow path.

According to the present invention, it is possible to provide the centrifugal rotating machine that is capable of further reducing vibrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a centrifugal compressor according to a first embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a main portion of the centrifugal compressor according to the first embodiment of the present invention.

FIG. 3 is an enlarged cross-sectional view of a main portion of a centrifugal compressor according to a second embodiment of the present invention.

FIG. 4 is a perspective view of an impeller according to the second embodiment of the present invention.

FIG. 5 is an enlarged cross-sectional view of a main portion of a centrifugal compressor according to a third embodiment of the present invention.

FIG. 6 is a view when an impeller according to a third embodiment of the present invention is viewed in an axial direction.

FIG. 7 is an enlarged sectional view of a main portion of a centrifugal compressor according to a fourth embodiment of the present invention.

FIG. 8 is a view when an impeller according to a fourth embodiment of the present invention is viewed in the axial direction.

FIG. 9 is an explanatory view showing a flow of a fluid in a seal portion according to the fourth embodiment of the present invention.

FIG. 10 is a cross-sectional view showing a modified example of a protrusion portion according to each embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

A centrifugal compressor 100 (centrifugal rotating machine) according to a first embodiment of the present invention will be described with reference to the drawings. As shown in FIG. 1, the centrifugal compressor 100 includes a rotating shaft 1 which rotates around an axis O, a casing 3 which covers a periphery of the rotating shaft 1 to form a flow path 2, a plurality of stages of impeller 4 which are provided on the rotating shaft 1, and protrusion portions 9 which are provided in the casing 3.

The casing 3 is formed in a cylindrical shape extending along the axis O. The rotating shaft 1 extends to penetrate an inside of the casing 3 along the axis O. A journal bearing 5 and a thrust bearing 6 are respectively provided on both end portions of the casing 3 in a direction of the axis O. The rotating shaft 1 is rotatably supported around the axis O by the journal bearing 5 and the thrust bearing 6.

An intake port 7 through which air serving as a working fluid G is taken in from the outside is provided on a first side (one side) of the rotating shaft 1 of the casing 3 in the direction of the axis O. In addition, an exhaust port 8 through the working fluid G compressed inside the casing 3 is exhausted is provided on a second side (other side) of the rotating shaft 1 of the casing 3 in the direction of the axis O.

An internal space which communicates with the intake port 7 and the exhaust port 8 is formed inside the casing 3. An inner diameter of the internal space repeatedly increases and decreases. The internal spaces accommodate a plurality of impellers 4 and form a portion of the flow path 2. Moreover, in following descriptions, a side on which the intake port 7 is located in the flow path 2 is referred to as an upstream side, and a side on which the exhaust port 8 is located is referred to as a downstream side. A return vane 50 is provided on the downstream side of each impeller 4 on the flow path 2.

In the rotating shaft 1, the plurality of (six) impellers are provided on an outer peripheral surface of the rotating shaft 1 at intervals in the direction of the axis O. As shown in FIG. 2, each impeller 4 includes a disk-shaped disk 41 centered on the axis O, a plurality of blades 42 which are provided on an upstream surface of the disk 41, and a cover 43 which covers the plurality of blades 42 from the upstream side.

When viewed in a direction intersecting the axis O, an outer dimension of the disk in the radial direction is formed so as to gradually expand from the first side to the second side in the direction of the axis O, and thus, the disk 41 is formed in an approximately conical shape. A plurality of blades 42 are radially arranged radially outward about the axis O on a surface (disk upstream surface 41A) facing the upstream side of both surfaces of the disk 41 in the direction of the axis O. More specifically, each blade is formed by a thin plate erected from the disk upstream surface 41A toward the upstream side. When viewed in the direction of the axis O, the plurality of blades 42 is curved to be directed from one side from the other side in a circumferential direction with respect to the axis O.

A surface (disk back surface 41B) facing the downstream side of both surfaces of the disk 41 in the direction of the axis O expands radially outward with respect to the axis O from the rotating shaft 1. The disk back surface 41B and the casing 3 (casing facing surface 3B) are disposed with a gap in the direction of the axis O. This gap is formed along a direction perpendicular to the direction of the axis O.

An upstream end edge of the blade 42 is covered with the cover 43. In other words, the plurality of blades 42 are interposed between the cover 43 and the disk 41 in the direction of the axis O. Accordingly, a space is formed by the cover 43, the disk 41, and a pair of blades 42 adjacent to each other. This space forms an impeller flow path 21 which is a portion of the above-described flow path 2. Moreover, in the following descriptions, a radially inner end portion of the impeller flow path 21 is referred to as an inlet 21A, and a radially outer end portion thereof is referred to as an outlet 21B. An outer peripheral surface (cover outer peripheral surface 43A) of the cover 43 extends radially outward toward the second side in the direction of the axis O, and thus, is formed in an approximately conical shape.

The cover outer peripheral surface 43A faces an inner peripheral surface (casing inner peripheral surface 3A) of the casing 3 with a gap. The casing inner peripheral surface 3A extends radially outward from the first side toward the second side in the direction of the axis O, following a shape of the cover outer peripheral surface 43A. An outer flow path F is defined between the casing inner peripheral surface 3A and the cover outer peripheral surface 43A. In the following descriptions, an end portion side corresponding to a side of the outlet 21B of the above-described impeller flow path 21 in an extension direction of the outer flow path F is simply referred to as an “outlet side”, and an end portion side corresponding to a side of the inlet 21A is simply referred to as an “inlet side”.

An annular space centered on the axis O is formed radially inside the casing inner peripheral surface 3A. This space is referred to as a cavity C. A seal portion S is provided on a first side (upstream side) of the cavity C in the direction of the axis O. The seal portion S is provided to seal leakage of a fluid between the casing 3 and the cover outer peripheral surface 43A. The seal portion S has a plurality of seal fins S1 and a base portion S2 which supports the seal fins S1.

A plurality of protrusion portions 9 for guiding the fluid flowing into the outer flow path F are provided in the outer flow path F. Each protrusion portion 9 protrudes from the casing inner peripheral surface 3A toward the second side in the direction of the axis O and extends from the outlet side toward the inlet side. The plurality of protrusion portions 9 are arranged at intervals in a circumferential direction with respect to the axis O in the outer flow path F. Each protrusion portion 9 is formed in a plate shape which extends from the outlet side toward the inlet side. Moreover, in the present embodiment, the protrusion portion 9 is provided at a position overlapping the outlet 21B of the impeller 4 in the radial direction with respect to the axis O. In other words, when viewed in the direction of the axis O, the protrusion portion 9 is provided at a position overlapping the outlet 21B of the impeller 4. Moreover, in a cross-sectional view including the axis O, the protrusion portion 9 is formed in a rectangular shape.

Next, an operation of the centrifugal compressor 100 according to the present embodiment will be described. When the centrifugal compressor 100 is operated, first, the rotating shaft 1 is rotationally driven by a drive source such as an electric motor. As the rotating shaft 1 rotates, the impellers 4 rotate, and the working fluid G is introduced from the intake port 7 into the flow path 2. The working fluid G introduced into the flow path 2 is sequentially compressed while passing through the impeller flow path 21 in each impeller 4. The working fluid G that has been compressed and brought into a high pressure state is pumped to the outside via the exhaust port 8.

Meanwhile, as indicated by a broken line arrow in FIG. 2, in the outer flow path F, the high pressure working fluid G may flow in from the outlet 21B side of the impeller flow path 21. A swirl component (swirling flow component) accompanying rotation of the impeller 4 is added to the fluid flowing through the outer flow path F. The swirl component turns in the same direction as a rotation direction of the impeller 4. Due to an influence of this swirl component, an excitation force directed in a direction orthogonal to a displacement direction acts on impeller 4. A swing-rotation vibration may occur in the rotating shaft 1 and the impeller 4 by continuously adding this excitation force.

However, according to the above configuration, the working fluid G which has flowed into the outer flow path F is guided by the protrusion portion 9 provided on the inner peripheral surface (casing inner peripheral surface 3A) of the casing 3. The protrusion portion 9 extends from the outlet 21B side of the impeller 4 toward the inlet 21A side thereof in the outer flow path F. Therefore, even in a case where the working fluid G includes the swirl component, the working fluid G is guided by the protrusion portion 9, and thus, the swirl component can be reduced. As a result, the possibility of the swing-rotation vibration occurring in the rotating shaft 1 and the impeller 4 can be reduced.

Furthermore, according to the above configuration, the protrusion portion 9 is provided at the position overlapping the outlet 21B of the impeller 4 in the radial direction. Accordingly, the swirl component included in the working fluid G which has flowed into the outer flow path F can be reduced immediately after the inflow. In particular, in the outer flow path F, the closer the outlet 21B of the impeller flow path 21 is, the more the swirl component is. Therefore, the swirl component can be more effectively reduced by the above configuration. As a result, it is possible to further reduce the possibility of the impeller 4 being displaced or excited due to the influence of the swirl component.

Further, as compared to a configuration in which the protrusion portion 9 extends over the entire area of the outer flow path F, by providing the protrusion portion 9 in a portion of the outer flow path F, only a necessary and sufficient amount of the swirl component is removed. Accordingly, it is also possible to avoid an increase in a frictional resistance between the cover outer peripheral surface 43A of the impeller 4 and the working fluid G due to an excessive reduction of the swirl component. If the swirl component is excessively reduced, a flow velocity of the working fluid G flowing in the outer flow path F becomes too small. Therefore, the frictional resistance due to the working fluid G increases between the cover outer peripheral surface 43A of the impeller 4 and the casing inner peripheral surface 3A, and a smooth rotation of the impeller 4 may be hindered. According to the configuration, the possibility can be reduced.

Moreover, according to the configuration, the plurality of protrusion portions 9 are formed at intervals in the circumferential direction. Accordingly, the swirl component can be uniformly reduced over the entire area of the outer flow path F in the circumferential direction. Thereby, a pressure distribution of the fluid in the outer flow path F becomes uniform, and thus, vibrations generated in the rotating shaft 1 and the impeller 4 can be further effectively suppressed.

Hereinbefore, the first embodiment of the present invention is described. In addition, various changes and modifications can be made to the configuration without departing from the gist of the present invention.

Second Embodiment

Next, a second embodiment of the present invention will be described with reference to FIGS. 3 and 4. In addition, the same reference numerals are assigned to the same configurations as those of the first embodiment, and detail descriptions thereof are omitted. As shown in FIGS. 3 and 4, in the present embodiment, a position of a protrusion portion 9B is different from that in the first embodiment, and a step 10 is provided on the cover outer peripheral surface 43A closer to the outlet 21B side than the protrusion portion 9B. FIG. 4 is a view showing a state where only the impeller 4 is extracted. In a centrifugal compressor according to the present embodiment, a casing 3 is attached so as to cover an outside of the impeller 4, and the outer flow path F is formed between the impeller 4 and the casing 3. The rotating shaft 1 is attached to the impeller 4 so as to penetrate a center hole of the impeller 4, and the impeller 4 rotates together with the rotating shaft 1. The casing 3 is a case which covers the whole and is a member that does not rotate (stationary member). It is naturally necessary to form a gap between the casing 3 and the impeller 4, and thus, the outer flow path F is necessarily formed.

The step 10 protrudes from the cover outer peripheral surface 43A toward the first side in the direction of the axis O. The step 10 has an annular shape centered on the axis O. In addition, the step 10 is arranged closer to the outlet 21B side than the protrusion portion 9B when viewed in a cross section perpendicular to the axis O. Moreover, the protrusion portion 9B and the step 10 are arranged with an interval. More specifically, when viewed from the direction in which the outer flow path F extends, the protrusion portion 9B and step 10 overlap each other. Further, a cross section of step 10 is rectangular. A cross-sectional shape of step 10 may be a triangle or a trapezoid in addition to a rectangle.

According to the configuration, the step 10 is provided on the cover outer peripheral surface 43A of the impeller 4. Therefore, the working fluid G (broken line in FIG. 3) flowing out from the outlet 21B of the impeller flow path 21 is obstructed by the step 10, and thus, the working fluid G passes through a radially outer portion of the outer flow path F. That is, more fluid flows along the casing inner peripheral surface 3A where the protrusion portion 9B is provided. As a result, more fluid is guided toward the protrusion portion 9B. Thereby, the swirl component of the working fluid G which flows into the outer flow path F can be more positively reduced. Therefore, the possibility of vibrations occurring in the rotating shaft 1 and the impeller 4 can be further reduced.

Hereinbefore, the second embodiment of the present invention is described. In addition, various changes and modifications can be made to the configuration without departing from the gist of the present invention. For example, in the second embodiment, the configuration in which the protrusion portion 9B and the step 10 are provided is described. However, the protrusion portion 9B may not necessarily be provided, and a configuration in which only the step 10 is provided on the cover outer peripheral surface 43A of the impeller 4 may be adopted. According to this configuration, since the step 10 is provided on the cover outer peripheral surface 43A of the impeller 4, the gap between the cover outer peripheral surface 43A and the casing inner peripheral surface 3A can be reduced. That is, the amount of working fluid G flowing into the outer flow path F can be limited. As a result, it is possible to reduce the excitation force to the impeller 4 generated when a large amount of working fluid G flows into the outer flow path F.

Third Embodiment

Next, a third embodiment of the present invention will be described with reference to FIGS. 5 and 6. In addition, the same reference numerals are assigned to the same configurations as those of each of the embodiments, and detail descriptions thereof are omitted. As shown in FIG. 5 or FIG. 6, in the present embodiment, a shape of a protrusion portion 9C is different from those of the embodiments. The protrusion portion 9C extends over the entire area from the outlet 21B to the inlet 21A of the impeller flow path 21 in the outer flow path F. More specifically, the protrusion portion 9C extends from the outlet 21B of the impeller flow path 21 to an end portion on the second side of the cavity C in the direction of the axis O. The protrusion height (protrusion dimension from the casing inner peripheral surface 3A) of the protrusion portion 9C is constant over the entire area in the extension direction of the protrusion portion 9C. Furthermore, as shown in FIG. 6, the protrusion portion 9C is curved from a first side toward a second side in a circumferential direction with respect to the axis O as the protrusion portion 9C goes from the inlet 21A side toward the outlet 21B side. In other words, the protrusion portion 9C is curved in a curved shape which is convex toward a front side in the rotation direction of the impeller 4. Further, the protrusion portion 9C extends in a direction intersecting a radial direction with respect to the axis O (broken line in FIG. 6).

According to the above configuration, since the protrusion portion 9C is provided over the entire area of the outer flow path F, the swirl component of the working fluid G which has flowed into the outer flow path F can be further reduced. As a result, the possibility that vibrations occur in the rotating shaft 1 and the impeller 4 can be further reduced.

Here, a swirl component which turns from the second side toward the first side in the circumferential direction (that is, toward front side in rotation direction of the impeller) is added to the fluid flowing into the outer flow path F. According to the configuration, the protrusion portion 9C is curved from the first side toward the second side in the circumferential direction as the protrusion portion 9C goes from the inlet side toward the outlet 21B. That is, the protrusion portion 9C is curved in a direction opposite to a turning direction of the swirl component. Therefore, the swirl component can be rectified in the opposite direction by the protrusion portion 9C. As a result, it is possible to further reduce the possibility of the impeller 4 being displaced or excited due to the influence of the swirl component.

Hereinbefore, the third embodiment of the present invention is described. In addition, various changes and modifications can be made to the configuration without departing from the gist of the present invention.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be described with reference to FIGS. 7 to 9. In addition, the same reference numerals are assigned to the same configurations as those of each of the embodiments, and detail descriptions thereof are omitted. As shown in FIGS. 7 and 8, in the present embodiment, a shape of a protrusion portion 9D is different from that of each of the embodiments. The protrusion portion 9D is twisted from a first side toward a second side in a circumferential direction with respect to axis O with reference to an end edge 91 on the casing inner peripheral surface 3A side as the protrusion portion 9D goes from the outlet 21B side to the inlet 21A side. More specifically, while the end edge 91 extends linearly along the casing inner peripheral surface 3A, an end edge 92 on a side opposite to the end edge 91 is curved from the first side toward the second side along an arc centered on the end edge 91 as the end edge 92 goes from the outlet 21B side toward the inlet 21A side. In other words, as shown in FIG. 8, the end edge 92 is twisted from a front side toward a rear side (that is, side opposite to turning direction of swirl component included in flow in outer flow path) in a rotation direction R of the impeller 4 as the end edge 92 goes from the outlet 21B side toward the inlet 21A side. Therefore, the angle formed between the protrusion portion 9D and the casing inner peripheral surface 3A is larger as the angle is closer to the outlet 21B side, and is smaller as the angle is closer to the inlet 21A side.

According to the configuration, since the twist of the protrusion portion 9D is small on the outlet 21B side, the protrusion portion 9D has a large angle with respect to the casing inner peripheral surface 3A. Therefore, the flow of the working fluid G including the swirl component which has flowed into the outer flow path F from the outlet 21B side (that is, upstream side of outer flow path F) can be captured more efficiently. Thereby, the swirl component can be further reduced. Further, since the twist of the protrusion portion 9D is large on the inlet 21A side (that is, downstream side of outer flow path F), the protrusion portion 9D has a small angle with respect to the casing inner peripheral surface 3A. Therefore, the fluid guided by the protrusion portion 9D flows in a region biased in the vicinity of the casing inner peripheral surface 3A. As a result, the flow into the seal fin S1 on the inlet 21A side of the impeller 4 becomes a radially inward flow state (down flow) from the casing inner peripheral surface 3A to the cover outer peripheral surface 43A, and thus, a contraction effect in the seal fin S1 increases. Therefore, the working fluid G can flow more positively toward the seal fin S1 itself rather than toward a clearance V1 between the seal fin S1 provided on the upstream side of the casing inner peripheral surface 3A and the impeller 4 (cover outer peripheral surface 43A) (refer to FIG. 9). In other words, an apparent clearance V2 of the seal fin S1 can be made smaller than the actual clearance V1. Thereby, a leak flow which passes through the seal fin S1 can be further reduced.

Hereinbefore, the fourth embodiment of the present invention is described. In addition, various changes and modifications can be made to the configuration without departing from the gist of the present invention. For example, as a modification common to the embodiments, a configuration as shown in FIG. 10 can be adopted. In the example of FIG. 10, the protrusion portion 9 (9B, 9C, 9D) has a tapered shape such that a dimension thereof in a circumferential direction gradually decreases as the protrusion portion is separated radially inward from the casing inner peripheral surface 3A. According to this configuration, for example, even in a case where the outer peripheral surface of the impeller 4 (cover 43) comes into contact with the protrusion portion 9 (9B, 9C, 9D), a contact area between the protrusion portion 9 (9B, 9C, 9D) and the outer peripheral surface can be suppressed to be small. As a result, it is possible to prevent the impeller 4 from being damaged or from being vibrated.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 

What is claimed is:
 1. A centrifugal rotating machine comprising: a rotating shaft which extends along an axis; an impeller which is configured to pump a fluid flowing in from one side in an axial direction to an outside in a radial direction, the impeller including a disk fixed to the rotating shaft and a cover covering a blade provided on the disk; and a casing which accommodates the impeller, wherein an impeller flow path through which the fluid is pumped is formed by an upstream surface of the disk in the axial direction and an inner peripheral surface of the cover, wherein an outer flow path is formed by an outer peripheral surface of the cover and an inner peripheral surface of the casing facing the outer peripheral surface of the cover, wherein the outer flow path is connected to the impeller flow path at an outlet of the impeller flow path, and wherein a protrusion portion protruding from the inner peripheral surface of the casing is provided in the outer flow path.
 2. The centrifugal rotating machine according to claim 1, wherein a plurality of the protrusion portions are formed at intervals in a circumferential direction.
 3. The centrifugal rotating machine according to claim 1, wherein the protrusion portion is provided at a predetermined region on the inner peripheral surface of the casing.
 4. The centrifugal rotating machine according to claim 1, wherein the protrusion portion is provided at a position overlapping the outlet of the impeller flow path in a radial direction with respect to the axis.
 5. The centrifugal rotating machine according to claim 1, further comprising: a step which is provided on the outer peripheral surface of the cover and is an annual step centered on the axis in the outer flow path, wherein the step is provided radially outside the protrusion portion.
 6. The centrifugal rotating machine according to claim 1, wherein the protrusion portion is provided over the entire area of the casing inner peripheral surface in the outer flow path.
 7. The centrifugal rotating machine according to claim 1, wherein the protrusion portion is curved from one side toward the other side in a circumferential direction with respect to the axis as the protrusion portion goes outward in the radial direction.
 8. The centrifugal rotating machine according to claim 1, wherein the protrusion portion is twisted from one side toward the other side in a circumferential direction with respect to the axis with reference to the inner peripheral surface of the casing as the protrusion portion goes inward in the radial direction.
 9. The centrifugal rotating machine according to claim 1, further comprising: a seal portion which is provided on a radially inner end portion of the outer flow path and seals leakage of the fluid between the inner peripheral surface of the casing and the outer peripheral surface of the cover.
 10. The centrifugal rotating machine according to claim 8, wherein the protrusion portion is twisted from a front side toward a rear side in a rotation direction of the impeller with reference to the inner peripheral surface of the casing as the protrusion portion goes inward in the radial direction.
 11. The centrifugal rotating machine according to claim 1, wherein a circumferential dimension of the protrusion portion with respect to the axis gradually decreases as the protrusion portion is separated from the inner peripheral surface of the casing.
 12. A centrifugal rotating machine comprising: a rotating shaft which extends along an axis; an impeller which is configured to pump a fluid flowing in from one side in an axial direction to an outside in a radial direction, the impeller including a disk fixed to the rotating shaft and a cover covering a blade provided on the disk; and a casing which accommodates the impeller, wherein an impeller flow path through which the fluid is pumped is formed by an upstream surface of the disk in the axial direction and an inner peripheral surface of the cover, wherein an outer flow path is formed by an outer peripheral surface of the cover and an inner peripheral surface of the casing facing the outer peripheral surface of the cover, wherein the outer flow path is connected to the impeller flow path at an outlet of the impeller flow path, and wherein a step which is an annual step centered on the axis is provided on the outer peripheral surface of the cover in the outer flow path. 